In recent years, there have been developed printed wiring boards, in which a circuit pattern is formed on an insulating substrate, such as a plastic substrate, a ceramic substrate or a metallic substrate having thereon a coating layer formed from plastic or the like. Formation of an electronic circuit from such a printed wiring board typically employs a technique of soldering electronic parts, such as IC elements, semiconductor chips, resistors and capacitors, onto the circuit pattern.
When such a technique is employed, the process for joining a lead terminal of an electronic part to a predetermined area of the circuit pattern typically proceeds as follows: forming in advance a solder thin layer on a surface of a conductive circuit electrode provided on the substrate; applying a solder paste or a flux through printing; positioning and placing an electronic part of interest; and reflowing the solder thin layer or both the solder thin layer and the solder paste.
In a recent trend for the miniaturization of electronic products, demand has arisen for fine-pitch patterning of solder circuit boards. On such solder circuit boards, fine-pitch parts such as a 0.3-mm-pitch QFP (Quad Flat Package)-type LSI and CSP (Chip Size Package) and a 0.15-mm-pitch FC (Flip Chip) are mounted. Thus, solder circuit boards are required to have a minute solder circuit pattern which allows fine-pitch mounting.
When a solder circuit is formed on a printed wiring board by use of solder film, a method such as plating, the hot air leveler (HAL) method or a combination of printing solder powder paste and reflowing is employed. However, plating has a drawback in that a thick solder layer is difficult to form, and the HAL method and printing of solder paste encounter difficulty in provision of fine-pitch patterns.
In order to overcome the aforementioned drawbacks, a method for forming a solder circuit without requiring cumbersome operations such as positioning of a circuit pattern is disclosed (see, for example, JP-A HEI 7-7244). In the method, a surface of a conductive circuit electrode provided on a printed wiring board is reacted with a tackiness-imparting compound so as to impart tackiness to the surface, solder powder is deposited on the thus formed tacky area, and the printed wiring board is heated so as to melt the solder, whereby a solder circuit is formed.
Employment of the method disclosed in JP-A HEI 7-7244 enables a minute solder circuit pattern to be formed through a simple operation, thereby providing a circuit board with high reliability. However, since the method includes depositing solder powder on a circuit board under dry conditions, the solder powder may be deposited on an undesired area or may fly off due to static electricity, impeding formation of a fine-pitch circuit board, and, problematically, the solder powder cannot be efficiently utilized. These problems are more detrimental particularly when solder micro-powder is employed.
In an attempt to solve the aforementioned problems, an object of the present invention is to provide, on the basis of the method disclosed in JP-A HEI 7-7244 for producing a solder circuit board in which a surface of a conductive circuit electrode provided on a printed wiring board is reacted with a tackiness-imparting compound so as to impart tackiness to the surface, solder powder is deposited on the thus formed tacky area, and the printed wiring board is heated so as to melt the solder, whereby a solder circuit is formed, a method for producing a solder circuit board which allows realization of a further minute circuit pattern. Another object is to provide a solder circuit board having a minute circuit pattern and high reliability. Still another object is to provide an electronic-part-mounted electronic circuit component which realizes high reliability and high mounting density.
The present inventors have carried out extensive studies in order to solve the aforementioned problems, and have achieved the present invention. Accordingly, the present inventors have developed the following techniques through which the problems have been solved.